1. Field of the Invention
The invention is directed to a circular cooling tower with heat exchangers of the dry-type, operating by natural draft and achieving the exchange of heat between two fluids such as atmospheric air, ordinarily, and another fluid, generally water.
2. Description of the Prior Art
Heat exchangers for cooling towers are generally rectangular units (parallelipipedic) of smooth or finned tube batteries joined at their extremities by fluid-feed boxes, and the fluid which circulates from one box to the other being cooled by the cold air which crosses the interstices between the tubes. The material of the metal or plastic tubes is selected in accordance with the nature of the warm fluid to be cooled; the air emitted by the tower is generally hot and dry, e.g., air at 40.degree. C. with 15% relative humidity.
Such cooling towers, in the form of a chimney and, having at their base, an air intake entry, surmounted by the lintel of the tower serving as a seating for the chimney, are generally for the purpose of cooling a fluid, generally the water from steam-turbine condensers of electrical power plants, or for condensing directly the water vapor originating from the turbines and cooling the hot condensate. The electrical power produced is related with the cooling power of the tower; that is, among other things, to the total length of tubes of the exchangers, but the efficiency of the exchange of heat depends also on the uniformity of passage of the air through the heat exchangers.
The establishment and maintenance of optimum conditions pose thorny problems with regard to the dimensions of the tower, the arrangement of the batteries and the means to provide for minimizing the harmful effects of the wind on the heat exchangers.
Since the cold air surrounding the tower enters the tower horizontally through the air intake opening and escapes vertically through the chimney, two distinct mountings of the exchange surface can be designed. In accordance with the first, the units are placed in the opening itself with their tubes erected vertically; according to the other, the units are placed across a section of the chimney, at its base, with their tubes aligned substantially horizontally.
The principal advantage of vertical-battery towers, resides in the simplicity of the mounting. On the other hand, such towers are extremely sensitive to variations in the speed and direction of the wind, since the units exposed to the wind are cooled more than those which are located out of the wind and there are sometimes local observations of inversions in the direction of the air flow through the tower. It also happens, during a storm, that the air passes right through the units on the periphery of the tower. It is quite obvious that the local and overall fluctuations in the cool air current crossing the tower and emitted by the chimney cause cooling irregularities for the water and are inevitably translated by changes in electrical power output. It is also clear that these fluctuations cannot be totally prevented, even if the wind is constantly moderate.
It has been proposed to minimize the effect of the wind by placing, in front of the openings, at the base of the tower mobile panels adjustable according to the direction and intensity of the wind. Such means makes it possible to improve the overall yield of the tower, but the cost of installing the panels and, especially, of their operation takes a heavy toll on the expense of the electrical plant.
The second type of mounting which can be designed for the dry surface of heat exchange units consists in equipping the section of the chimney situated just above the air intake opening with horizontal or slightly inclined batteries or units aligned for the most part radially or concentrically in relation to the periphery of the chimney.
In such towers it is known that the amount of cold air entering into contact with the exchangers is different at different distances from the center, so that the exchange of heat is irregular and its yield less satisfactory. Such an arrangement of exchangers, then, does not produce the best overall exchange of heat, even if the wind effect is not considered; while the cooling yield worsens when the wind blows.
It has been suggested that the cold air flow profile could be improved by directing the air along directing surfaces (air deflectors) having low resistance to the passage of the air, and which are distributed over the entire section of the tower, but this method is quite costly.
It is also known to compensate for the harmful effects by reducing the height to which the cold air is brought to the batteries located in the center of the tower, i.e., by placing ceter units at a lower level than the peripheral units, but this arrangement makes the central units much more sensitive to the effect of the wind.
A prior art answer to this last mentioned disadvantage was proposed and comprises in placing the central units higher than the peripheral units, which results in reducing the draft height of the chimney.
Lastly, a prior art compromise between the various previous proposals, consists in mounting the units in hog-back fashion with separating partitions in order to use them as air-guides, in aligning them progressively and radially downwards from the periphery towards the center of the tower, and in imparting to the units a height which progressively decreases toward the center in order to regularize the cold air flow profile without compromising the draft of the chimney.
It is likely that the arrangement recommended does indeed improve the overall yield of the tower but the use of units which have different dimensions, calculated according to the position which they are supposed to occupy in the tower, and which are provided with separating partitions for the purpose of blocking the wind, will of necessity increase the cost of the installation. In addition, with both types of arrangements of units as mentioned above, it would appear that the cooling power can only be increased by increasing the diameter of the tower, which in turn increases the cost of construction.